There are several AAVSO comp stars present in a 30' FOV of DW UMa. In VPhot, open one of your DW UMa images. Under catalogs, click add AAVSO Comp stars. The comp stars will be displayed on your image. Select as many as you want (single comp or ensemble). Highlight one of the comps and change it to a check star.

If you have such a small FOV that only one AAVSO comp star exists, try to load simbad data and see if a star appears near the target. Use it as the check.

If not, select any other star in your FOV and estimate its magnitude by SNR. Input an estimated magnitude for the star and VPhot will use this as the check. Run your time series. Obviously, the magnitude of this check star will not be accurate but you can run the time series.